Process for porous materials and property improvement methods for the same

ABSTRACT

A powder metallurgy method of forming a lightweight porous material body that includes mixing matrix material powder with one or more types of high volatility material powder having tendency to vaporize when heated to its vaporization temperature; hot consolidating the mixture under sufficient pressure and at a temperature below vaporization temperature of high volatility material powder to form a substantially consolidated body consisting of dispersions of high volatility material powder particles in the matrix material; heating the consolidated body to a temperature sufficient to vaporize the high volatility material powder particles and to create a vapor pressure sufficiently high to cause yielding of surrounding matrix material, thus forming pores, cooling the compact body after pore formation.

BACKGROUND OF THE INVENTION

The present application claims priority to U.S. Provisional Patent Application No. 60/685,519, filed May 31, 2005, and U.S. Provisional Patent Application No. 60/697,932, filed Jul. 11, 2005, each of which are specifically herein incorporated by reference in their entirety.

The present invention is directed to a process of forming lightweight, and high strength and lightweight porous materials. In one embodiment, the present invention offers a process to dispersion harden materials by forming nano and micrometer sized pores with thermally stable compounds formed on pore surfaces. Pores thus formed are small and spaced in short proximity to each other sufficiently to act as barriers to dislocation movement, strengthening the material, while lowering its density. In another embodiment, the invention provides a method of manufacturing parts with internal cavities, parts being shaped by vapor pressure from within.

This invention relates to the creation of pore-strengthened metals (and other materials) that can have two to three times the strength of conventional solid metals per unit weight. This development can lead to significant reductions in the weights of air and land transportation vehicles.

As the fuel costs increase, the importance of land vehicle weight reduction becomes more urgent. A 10% savings in the weight of a passenger car can translate into a 7% savings in fuel. 7% savings equals, in todays near $60 per barrel oil prices, $33 billion savings in the U.S. national annual petroleum costs, and reduction of the trade deficit by twice that amount, every year. In air travel the effect could even be more significant. This is an indication of the vast market that this invention can create, savings billions in energy costs.

Additional applications of the invention include production of metal, ceramic, glass, and plastic filters, sound and energy absorbing parts, medical devices and implants, battery and hydrogen fuel cell electrodes, ductile glass, ductile ceramics, and thermally insulated structural parts.

PRIOR ART

There are no directly comparable prior art that dispersion strengthens materials by creating oxidized pores. However, a number of patented processes refer to creation of metal foams, containing pores typically too large and too uncontrollable to be useful in material strengthening. Generating a gas in a liquid generally creates metal foam. The gas can be injected from a gas supply, foaming agents such as hydroxides and carbonates can be added, or impellers can be used to foam the metal. It is also known in the art to provide the molten metal with a number of stabilizer additives to assist in the foaming process and to maintain the stability of the formed pores (cells).

In U.S. Pat. Nos. 5,221,324 and 5,622,542 for example, a composite of a metal matrix, e.g. aluminum, and finely divided stabilizer particles such as silicon carbide, is heated above the liquidus temperature of the metal matrix and this is mixed such that a vortex is formed. The molten composite is blanketed with a gas and during the vortex mixing. This gas is drawn into the melt to produce an expanded, viscous molten composite material containing pores. And, in the U. S. Pat. No. 4,973,358, instead of causing a vortex, gas bubbles are discharged into the liquid metal below the surface. There are other variations of the same idea. In all, the common approach is to create gas pores while the matrix material is in a liquid state. This approach, therefore, is not very suitable for creation of pores with uniform size and distribution, such as the pores that can be used in strengthening the matrix material. The bubble formation is too uncontrollable, and pore sizes created can only be too large to be effective in dispersion strengthening. Additionally, working with molten metals is very corrosive to the containers, and apparatus that are used to blow or to rotate the melt.

The U.S. Pat. No. 3,087,807 starts with powder mixtures, containing a foaming agent, to manufacture porous aluminum. In this method, a mixture of a metal powder and calcium carbonate (or zirconium hydride or titanium hydride) is cold-compacted and later extruded to form an aluminum rod. The extruded rod thus produced can be foamed to form a porous metal body by heating it at least to the melting point of the metal. However, heating rate has to be very high to generate the foaming action. The patent limits the heating rate to a range of 20 to 130 degrees Fahrenheit per second (or 1200 to 7200 degrees Fahrenheit per minute). This is extremely fast heating and is impractical for creating pores of substantially uniform size in any rod material having a thickness that is meaningful in industry. Such fast heating rates eliminate typical industrial furnaces from being used in the process, and require expensive heating equipment, such as laser, electron beam, or induction heating, thus making the process expensive. Additionally, this method involves two-step compaction of powders, one of the steps being an extrusion step. Thus, is unnecessarily expensive.

In the U.S. Pat. No. 5,151,246, a method is described “for manufacturing foamable metal bodies in which a mixture of a metal powder and a gas-splitting propellant powder is hot-compacted to a semi finished product at a temperature at which the joining of the metal powder particles takes place primarily by diffusion and at a pressure which is sufficiently high to hinder the decomposition of the propellant in such fashion that the metal particles form a solid bond with one another and constitute a gas-tight seal for the gas particles of the propellant.”

This process is impractical, expensive, and is limited to lower melting point metals. The one reason for these deficiencies is the use of diffusion bonding as a means to form a gas tight seal for the gas particles of the propellant. Diffusion bonding under pressure requires some means to apply pressure, such as a press ram or a piston that is made of heat resisting materials that would not deform with time due to the metal creep. This adds to the cost of the process. Additionally, times required for the diffusion bonding of powder particles are by necessity too long, and as a consequence processing costs would be high.

A further unwanted consequence of diffusion bonding involves the potential degradation of the material properties due to a small inclusion, a foreign substance, which can affect properties by diffusing long distances within the solid structure of the matrix material, and creating a large volume of weakness in the material.

Another deficiency of this prior art process is its use of propellant materials that have decomposition temperatures below the hot compaction temperature. This comes about by the desire to use low-priced propellant materials, such as carbonates and hydrides. As a consequence, during the hot compaction, a constant pressure must be applied to prevent gasses from evolving from decomposition of the propellant. This approach limits the process to propellants too difficult to control due to their tendency to create excessive gas pressures at foaming temperatures, and hydrogen containing propellants are frequently corrosive to many types of matrix metal. Excessive gas pressures can be generated if the temperature difference between the decomposition temperature and the foaming temperature is too large.

A further difficulty with this prior art patent involves its impracticality. Hot compaction or rolling must be carried out under protective atmospheres or under partial vacuum to yield any satisfactory bonding of metal powder particles. This protection is not indicated here. If the hot compaction were carried out in air, metal particle surfaces would heavily oxidize, which would render the powder nearly impossible to bond to each other. Rolling process under protective atmosphere or in vacuum would especially be very expensive. Also, when hot compacting the powder mixture, or during rolling, creation of any significant temperature gradient within the powder mass would render colder portions of the powder to remain unbonded, causing the subsequent foaming treatment useless in those regions. Because of these reasons, the process as described in U.S. Pat. No. 5,151,246 would not economically produce a usable product.

In the present invention none of these deficiencies exist. Gas forming high volatility compound is selected to have vaporization temperature above the hot compaction temperature used to consolidate the matrix metal powder. No long-time diffusion is necessary to bond powder particles, as the hot compaction step takes only a few seconds. Hot compaction in the present invention takes place under either protective atmosphere or under partial vacuum. All heating occurs nearly isothermally, so that no significant thermal gradient is created within the matrix metal being heated. Heating rates to pore forming temperature need not be as high as the prior art specifies. In fact, heating rate is preferred to be slower than 1° C. per second near the pore formation temperature to prevent thermal gradients within the matrix metal. This is necessary to produce uniform size pores.

In addition, the present invention provides methods other than powder compaction to distribute the pore forming compound particles within a dense matrix material. These include modified versions of electrolytic plating, vapor deposition methods, high energy rate forming methods such as explosive forming, thermal spraying, and molten droplet deposition. Plating, explosive bonding, and low-temperature vapor deposition can incorporate high volatility material particles into matrix material at relatively low-temperatures, these high volatility materials can have decomposition temperatures lower than the temperature of the process (plating, explosive bonding, and low-temperature vapor deposition) used.

SUMMARY OF THE INVENTION

An object of this invention is to provide porous bulk material structures, and as integral layers on or within bulk materials.

Another object of this invention is to provide pores in materials in sizes and with mean separation distances sufficient to cause strengthening of the matrix material within which they are formed.

Another object of this invention is to provide a method of forming thermally and chemically stable compounds (oxide, nitride, carbide, boride, etc.) on pore surfaces in sizes and with mean separation distances sufficiently small to cause strengthening of the matrix material within which they are formed.

Another object of this invention is to provide strengthening of metals, ceramics, glasses and plastics due to pores in sizes and with mean separation distance between them to cause strengthening in selected portions of a metal, ceramic or plastic body.

Yet, another object of this invention is to provide increased ductility to brittle materials, such as metals, ceramics and glasses, by providing pores that act as dislocation sinks.

Yet, another object of this invention is to provide means to control electromagnetic radiation transmission, absorption, and reflection characteristics of transparent substances, including plastics, glasses and other transparent materials.

Another object of this invention is to provide substantially uniform size pores in materials.

Another object of this invention is to provide substantially uniform size pores in materials in sizes and with mean separation distances less than 10 μ to cause strengthening of the matrix material within which they are formed.

A further object of this invention is to provide a simple manufacturing process for objects having at least one internal cavity. The simplified process eliminates the need to join two or more pieces to manufacture a hollow part.

Another object of this invention is to provide objects with internal hollow sections or channels, made of metals, metal alloys, ceramics, glasses, plastics, and ceramic-metal composites.

It is yet another object of this invention to provide objects with internal hollow sections or channels in pre-selected patterns in three dimensions.

It is yet another object of this invention to provide open or closed pored structures side by side within objects with internal hollow sections or channels in three-dimensional pattern.

It is another object of this invention to provide objects, with internal hollow sections or channels, shaped by the use of rigid cavity dies.

A basic powder metallurgy (P/M) process is offered to manufacture porous materials involving mixing of a matrix material powder with a minor constituent, high volatility material powder, to form a composite mixture, and hot consolidating the mixture to an object to near full density at a temperature T1, where T1 is below the decomposition temperature of the high volatility material. The term “matrix material”, refers to the base material that acts as host to the high volatility material powder. Then, taking the consolidated object to a pore forming temperature T2, where T2>T1, and where, at T2, vapor pressure of the high volatility material is greater than the yield strength of consolidated powder of the matrix material. Pressure exerted on matrix material by the vapor generated by high volatility material particle causes the matrix material to yield and a gas pore is formed while the matrix material is still in solid state.

In one embodiment of the invention, the decomposing high volatility material particles create stable compound forming agents that can react with the matrix material to form thermally stable oxides, carbides, nitrides, or borides. For example, oxide-forming agents may be oxygen atoms and oxygen containing molecules, or oxidizing molecular compounds, which react with matrix material constituents to form stable oxides on and near pore surfaces. The pores are small enough (smaller than 100 μm) and in close proximity (closer than 10 μm) of each other to create stable barriers to dislocations within the matrix material, thus causing strengthening of the matrix material. In brittle materials, such as ceramics and glasses, and some metals, pores can act as dislocation sinks, rendering the material more ductile.

This invention in another embodiment, also provides a method of manufacture of objects with internal hollow sections and, or hollow channels; and, the manufacture of such objects can be accomplished basically by the following steps:

-   -   a. Distributing at least one high volatility material powder         particle within a substantially dense matrix material to create         a composite;     -   b. Heating said composite to a pore formation temperature, at         which vapor pressure of said high volatility material is higher         than the yield strength of said matrix material, wherein said         high volatility material particle evaporates to form at least         one cavity within said matrix material;

The process of distributing may involve a number of methods including one of the following methods modified to introduce fine particles of high volatility material in the matrix material:

-   -   Powder consolidation of a powder mixture under pressure,     -   Electrolytic plating,     -   Electro-less plating,     -   Molten droplet spraying,     -   Vapor deposition,     -   Plasma spraying and coating,     -   High-energy rate forming of powder mixture (i.e., explosive         forming, electro-hydraulic forming, cladding, magnetic forming,         etc.)     -   Roll forming of powder mixture,     -   Extruding of powder mixture,     -   Plastic forming,     -   Sintering of dried ceramic slurries, and slips,     -   Other similar methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a metal can lined with an insulation material, and filled with a mixture of the powders of the matrix material and high volatility material;

FIG. 2 is a cross-sectional view of a metal can lined, with an insulation material, and filled with solid metal section, on top of which is a layer of matrix material powder and high volatility material powder mixture;

FIG. 3 is a plan view showing materials of FIG. 1 or 2 inside a degassed and sealed metal can, being heated to a consolidation temperature (T1);

FIG. 4 is a plan view of the degassed, sealed and heated can of FIG. 3, being consolidated under pressure within a cavity die;

FIG. 5 is a cross-sectional view of a consolidated material block of FIG. 2 being rolled to a reduced thickness;

FIG. 6 is a plan view of a computer controlled x-y table that can be used to distribute high volatility material particles in three dimensions within the matrix powder mass;

FIG. 7 is the plan view of a degassed, sealed and heated can of FIG. 6, being consolidated under pressure within a cavity die;

FIG. 8 is a cross-sectional view showing a consolidated matrix material containing high volatility material particles within an open top mold;

FIG. 9 is a cross-sectional view of the consolidated matrix material of FIG. 8 expanded due to vapor pressures of high volatility material particles forming cavities within the matrix material after heating to pore formation temperature;

FIG. 10 is a cross-sectional view of a consolidated matrix material with high volatility material particles placed inside a closed mold cavity;

FIG. 11 is a cross-sectional view of the consolidated matrix material of FIG. 10 expanded due to vapor pressures of high volatility material particles forming cavities within the matrix material after heating to pore formation temperature, filling the mold cavity;

FIG. 12 is a cross-sectional view of a consolidated matrix material with high volatility material particles placed inside an irregularly shaped, closed mold cavity;

FIG. 13 is a cross-sectional view of the consolidated matrix material of FIG. 12 expanded due to vapor pressures of high volatility material particles forming cavities within the matrix material after heating to pore formation temperature, filling the irregularly shaped mold cavity;

FIG. 14 is a cross-sectional view of the consolidated matrix material of FIG. 12 expanded due to vapor pressures of high volatility material particles forming open and closed cavities within the matrix material after heating to pore formation temperature, filling a partially open mold cavity;

FIG. 15 is a cross-sectional view of a consolidated matrix material with high volatility material particles closely spaced in pattern within the matrix material in three dimensions, placed inside a open top mold cavity;

FIG. 16 is a three dimensional view of the open channels formed within the matrix material of FIG. 15 after heating to pore forming temperature;

FIG. 17 is a cross-sectional view showing a consolidated matrix material containing an high volatility material particle within a closed, irregularly shaped mold;

FIG. 18 is a cross-sectional view of the consolidated matrix material of FIG. 17 expanded to conform to the internal shape of the mold due to vapor pressure of the high volatility material particle after heating to pore formation temperature;

DETAILED DESCRIPTION

Most metal, plastic, glass or ceramic materials can be manufactured with high porosity content. And, if desired, these materials can be manufactured with integrally grown porous layer(s). Pore sizes can be selected to range from nanometer sizes to micrometer, or even millimeter and larger in scale. Pore walls can be oxidized and the distribution and size of pores can be controlled such that the oxide walled pores can become barriers to dislocation movement, thus strengthening the matrix metal in which they are imbedded.

Though, there can be several simple ways to manufacture such porous materials or layers, each of these manufacturing methods would rely on a simple rule for pore formation. That is, at an elevated temperature preferably below the melting point of the matrix material, the vapor pressure of the gaseous element or the gaseous molecular compound released by vaporization of the high volatility material particles must become higher than the yield strength of the matrix material. Vaporization can take place by four different mechanisms².

-   -   1. Vaporization to gas molecules of the same composition         (sublimation)         For example: ZrO₂(s)=ZrO₂(g)     -   2. Vaporization with formation of a polymer         For example: 3MoO₃(s)(MoO₃)₃(g)     -   3. Vaporization by decomposition to the elements         For example: BaS(s)=Ba(g)+S(g), CuO(l) =Cu(l)+½O₂(g)     -   4. Vaporization by disproportionation to dissimilar molecules,         which are not both elements, For example:         2TiBr₃(s)=TiBr₂(s)+TiBr₄(g), 4CeS(l)=Ce(g)+Ce₃S₄(l)

Where (g) means gas, (l) means liquid, and (s) means solid.

Vaporization may take place by several of these processes occurring simultaneously, in which case, the vapor pressure is the sum total of all the partial pressures of the gaseous molecules produced.

Among the four mechanisms of vaporization listed above, sample reactions of numbers 1, 2, and 3 have the potential of oxidizing (or forming other stable compounds such as nitrides, borides, and carbides) the walls of the pores formed, assuming that the matrix material is a material, such as a metal, that has affinity for oxygen, and that its reactivity is high enough to reduce the vaporized oxide gas. This may also be true for number 4 depending on the compound going through disproporionation.

Vaporization temperature is defined here as that temperature at which significant vapors form. At this temperature, for example, vapor pressure may reach up to 1 atmosphere.

For example, in a powder metallurgy (P/M) approach, a method of making porous body may involve mixing powder of the matrix material with a small amount of powder of the high volatility material, which is chosen for its high vapor pressure at elevated temperatures, and consolidating the mixture to form a composite object that has higher density, or reaches its full density at a temperature T1. Then, taking the consolidated object to a temperature T2, where T2>T1, and where, at T2, vapor pressure of high volatility material is greater than the yield strength of consolidated matrix material. Such thermal processing creates pores within the consolidated object of the matrix material. Here “elevated temperature” may roughly be 50-250° C. for plastics, 200-2000° C. for metals, 1000-1400° C. for glasses, and 1,200-4000° C. for ceramics.

Ordinarily, the yield strength of a solid material decreases with increasing temperature from near absolute zero to about T_(M)/4 or T_(M)/3 (where T_(M) is the melting point expressed in the absolute temperature scale), at which temperature it begins to decrease rapidly. Yield strength of metals drop off rapidly at temperatures near 0.5 T_(M). Consolidation temperature T1 can be above the yield strength drop off temperature, but the pore formation temperature T2 must be at a higher temperature than the consolidation temperature. For most materials the pore formation temperatures preferably can fall within 0.6 to 0.99 T_(M) for better pore size and distribution control. When larger pores are desired, pore formation temperature can be above the solidus and below the liquidus temperatures of the matrix material. But within this temperature range pore size and distribution will be determined to some degree by the actual temperature, which determines the percent solid of the matrix material. Above the liquidus temperature, pore size would be large, and uneven, except when the liquid matrix material is inside a cavity die and is constrained on all sides when its growth, due to forming of pores, reaches a predetermined percent growth. The same restricted space approach can be used for forming pores while the matrix material is still solid, or the temperature is below its solidus. Then, the pore size could be controlled to some degree. Pore size uniformity can be accomplished by isothermal heating to the pore forming temperature and by allowing only a shallow thermal gradient to form during heating.

A preferred powder metallurgy method of forming a lightweight porous material body, comprises mixing matrix material powder with one or more types of high volatility material powder having tendency to vaporize when heated to its vaporization temperature or above its vaporization temperature. The relative amounts and particle sizes for the powders being pre-calculated to achieve the desired pore size, size uniformity, and distribution. Once well mixed in a powder blender, the mixture 11 is placed in a metal can 12 as shown in FIG. 1. Here, a thermal insulation material 13 may surround the powder mixture. Ceramic powders, for example could be used as thermal insulation. Thermal insulation such as the one described here, may be necessary to create an isothermal consolidation condition later in the process. The can assembly 10 may then be either heated to the consolidation temperature under a protective atmosphere cover, or the can be evacuated and sealed to create a partial vacuum inside, and then heated to the consolidation temperature. A partial vacuum of 0.1 mm Hg or less is typically sufficient for most metals. Protective atmospheres include inert gases, hydrogen, nitrogen, cracked ammonia, and carbon monoxide, or mixtures thereof. The protective atmosphere and the partial vacuum are necessary to prevent oxidation of matrix material powder, unless the matrix material is a material that is already an oxide, and will go no further oxidation when heated to consolidation temperature.

The mixture may also be placed next to a rigid body or a powder mass that has no high volatility material. This is illustrated in FIG. 2 where a rigid body 25 is placed below the powder mixture 20 in can 22 lined with a thermal insulation material 23. Here the rigid body 25 may be a partially densified powder compact or a solid piece. It may be placed in other locations next to the powder mixture 20, or it may even entirely encase powder mixture 20.

The mixture 11 in FIG. 1 is then heated to the consolidation temperature and pressed under sufficient pressure to densify the matrix powder to a density at least about 90% of matrix material's theoretical density. Consolidation may be accomplished in a cavity die 42 in FIG. 4, under a force 45 applied via a press ram 43 to the canned powder mixture 31, at a temperature below vaporization temperature of high volatility material powder. The application of the force 45 is quick and once the desired pressure is reached, the pressure need not be held for any period of time necessary to allow time for diffusion bonding of the matrix powder particles. At this point, it should be mentioned that the consolidation to full density is typical and is accomplished preferably under high pressure, rather than low-pressure and extended time. The latter approach is too expensive, both from the press utilization time point of view and from the thermal damage to the equipment. This method of powder consolidation has been used for metals and for some ceramics with high degree of success. The hot pressing thus described defines the word “consolidation” as it is used in this invention, and can easily produce a solid body consisting of dispersions of high volatility material powder particles in a matrix material. The pressure needed for copper and copper alloy powders, for example, is typically above 30 ksi, at temperatures near or slightly above 550° C. The minimum pressure and temperature combinations for the consolidation of steels are in the range of at least 45 ksi and 1100° C.; and for nickel based alloys and titanium alloys 60 ksi and 1175° C. respectively. For aluminum alloys consolidation can occur under a pressure and temperature combination better than 30 ksi and about 400° C. or higher depending on the alloy. These pressure and temperature combinations can produce consolidated metals with densities equal or higher than 90% of their theoretical densities. Such densities seem to be needed for pore formation in the solid state without destroying the compacted body.

The next step involves heating the consolidated body to a pore forming temperature sufficient to vaporize the high volatility material powder particles and to create a vapor pressure sufficiently high to cause yielding of surrounding matrix material, and thus forming pores. Heating can be at any rate, except that it is preferred to slow the rate of heating near the pore formation temperature in order to prevent creation of excessive thermal gradients within the consolidated body. This precaution allows production of pores more uniform in size throughout the consolidated body.

The pore size, shape, composition and structure of pore walls will depend on the interplay of many factors tending to influence the pore formation force balance, and vice versa. Among these factors, solubility of the species generated, their reaction(s) with the matrix material, diffusivities, kinetic concerns, such as each vapor pocket competing with the next pocket for available space, time available for reactions, expansion of species upon heating, (super) plasticity of the matrix material, etc. can be counted. Simple experimentation can determine the exact temperature at which pores should be formed, and what should be the size and type of high vapor pressure particles.

The pore formation temperature is therefore, defined here as the temperature at which the high volatility material particles can develop sufficient pressure to cause yielding of matrix material to form pores of pre-determined desired size. This can mean heating the consolidated body at least to the vaporization temperature and then heating to a higher temperature to allow the desired vapor pressure to develop within each pore.

The basic pore formation process involves vaporization of small powder particles pre-distributed within a matrix material. The distribution of high volatility material powder particles in a substantially dense matrix material can be accomplished in many ways including powder metallurgy (P/M) methods, such as the P/M method just described, and also by deposition methods such as electro-plating, physical and chemical vapor deposition, and molten material droplet and plasma spraying. Additionally, high energy forming, sintering, and roll forming can be utilized for consolidation of powder mixtures. In the P/M, and the deposition methods, high volatility material powder can be distributed within the matrix material in a pre-determined pattern, to create pore patterns, where the pores are individually placed and not in contact with each other; or the particles may be sufficiently close to each other, so that the pores formed may connect to form channels of pre-determined size and design.

For ceramics, distribution of the high volatility material particles within a ceramic matrix can be accomplished by mixing the high volatility material powder while the matrix ceramic powder is in the slurry stage. The slurry can then be dried and sintered at a temperature below the vaporization temperature of the high volatility material. Once the matrix ceramic is sintered, without cooling from the sintering temperature, the temperature can be increased to the pore formation temperature. Hot pressing can also consolidate ceramics. In this case, the method would be similar to the P/M method described above for metals.

For glasses, distribution of the high volatility material particles can be accomplished either in raw ingredient mixing stage, or while the glass is in liquid state. Increasing the temperature of the liquid to the pore formation temperature for a predetermined time can produce the desired pores.

Between the consolidation step and the pore formation step the consolidated body need not necessarily be cooled to the room temperature, but can be. This is determined in practice by the fabrication set-up and the type of matrix material. However, the pressure applied in consolidation is quickly released, typically a few seconds after the desired maximum pressure is reached.

The next step is to cool the consolidated body after pore formation, to entrap the pores thus formed.

In the P/M method, the consolidated body can be heated to a pore forming temperature below the solidus or liquidus or even above the liquidus temperature of the matrix material, depending upon the degree of pore size and uniformity of pore distribution desired. Heating to below the solidus temperature of the matrix material may best control smaller sized pores. If larger pore sizes and relatively less pore size uniformity can be tolerated, then the pore formation can occur while the matrix material is in the liquid state.

The high volatility material can be selected from among common elements, single and double oxides, fluorides, carbonates, hydrides, sulfides, borides, nitrides, carbides, chlorides, and other compounds of common elements.

Placing the consolidated body in a cavity die with pre-determined dimensions to restrict its growth during pore formation can also be used as a method of controlling pore growth. This approach can also be useful in manufacturing of shaped parts by allowing the growth of the consolidated body due to pore formation, in pre-determined dimensions and directions.

After consolidation and before pore formation treatment, the consolidated body can be deformation processed for easy conversion to typical industrial material forms, including bar stock, plate, sheet, foil, wire, forging stock, and rolled or extruded material shapes.

For example bar stock can be cut to desired sizes for forging, and the consolidated body can be forged to a semi-finished shape before pore formation treatment. The final machining to finished size and shape can be accomplished either before or after the pore formation.

For example, FIG. 5 is a cross-sectional view of a consolidated material block of FIG. 2 being rolled to a reduced thickness. The consolidated matrix metal—high volatility material powder mixture 20 and the solid base 25 which has been bonded to the powder mixture 20 in the consolidation step, FIG. 4., are all being reduced in thickness by going through the rolls 51 of a rolling mill 50, to produce a thinner, but longer, sheet material incorporating a thinner consolidated powder mixture 53 and thinner solid base 55. Such a sheet material can then be sectioned to desired size and shape and heated to the pore formation temperature to convert the material into a porous bulk metal or bulk metal clad with a porous layer for the material of FIG. 5.

Consolidation of the powder mixture in a shaped hard or soft die can produce net or nearly net shaped product that can later be put through a pore forming treatment.

The consolidated matrix material plus a distribution of high volatility material powder particles within it may also be rolled to sheet or foil thickness before pore formation. The latter can be used as filters or separators after pore formation that allows pore-to-pore connection, or pores that traverse the thickness of the foil. Such foils can also be produced by any of the deposition techniques mentioned above.

In the same preferred P/M approach, the high volatility material chemical composition can be chosen intentionally to allow release of reactive vapors upon heating to pore forming temperature, to form stable compounds, including oxides, carbides, nitrides, and borides, with one or more elemental ingredients of the matrix metal on or near pore surface. When the pore sizes are chosen to be less than 100 microns (micrometers), and the average pore spacing is less than 10 micro-meters, preferably less than 3 micro-meters (see for example the article by N. J. Grant of MIT, in Trans. AIME 200, p. 247, 1954), the matrix material can experience a strengthening effect due to creation of the thermally stable compounds on the surfaces of pores acting as barriers to dislocation movement. This effect, which is well known by materials scientists familiar with strengthening mechanisms, may be quite pronounced, and in some cases doubling, tripling, or even quadrupling of the strength of the matrix material, especially if the matrix material is a metal. It has been known for at least 60 years that incorporating thermally stable particle barriers to dislocation movement in metals can strengthen metals. N. J. Grant of MIT has demonstrated that when Al₂O₃ particle spacing is reduced from 1 μ to 0.3 μ in aluminum matrix, room temperature tensile strength more than doubled. Reducing particle spacing further can triple or may even be quadruple the yield strength. It can be noted in the experiments described later, that the size of pores and the average distance between the pores can easily provide significant increases in the strength of metals, and other materials.

In a second embodiment of the invention, a deposition method of forming a lightweight porous metal body is provided.

The first step involves dispersing one or more types of high volatility material powder within a matrix metal being deposited onto a base material; powder having tendency to vaporize when heated to its vaporization temperature; to create a deposited body made of matrix metal, wherein high volatility powder is distributed.

There are several deposition techniques that can be modified to accommodate introduction of high volatility material powder particles into the stream of metal being deposited. These include, among others, electrolytic and electroless plating, vapor deposition techniques, molten droplet spraying methods, and plasma spraying methods.

Plating techniques offer a way of incorporating high volatility material powder particles into a matrix metal at or neat room temperature, which allows the use of high volatility material powders with low vaporization temperatures to be used. The vaporization temperature can be any temperature between the room temperature and the solidus of the matrix material for this approach. The high volatility material powder may be mixed into the plating solution, and gravity or the surface electrical charge of the particles tending to deposit these high volatility powder particles on the receiving electrode being plated.

Many vapor deposition techniques exist. In all, the common characteristic is creation of the vapor of the material being deposited. While the matrix metal is being deposited, particles of the high volatility material can be incorporated into the vapor stream of the deposited matrix metal. The incorporation can take place at lower temperatures, near the target (previously deposited matrix material) since these particles need not be evaporated. The process for the molten droplet spraying methods would be similar. Here, the molten droplets of the matrix metal, formed due to heating by a high-energy source such as a laser, an electron beam or a plasma beam, is deposited onto a target material, which may or may not be disposable. Again, the particles of the high volatility material powder can be introduced into the molten droplet stream, thus being incorporated into the solidifying molten matrix material. Because the speed of this process is very high, and the molten droplets freeze almost instantaneously after hitting the target, the high volatility material particles do not have time to undergo phase transition into vapor, and can be trapped within the matrix metal.

Such plating, vapor deposition, and high-energy deposition techniques can create uniform deposited layers of the matrix metal, with the high volatility material powder particles dispersed within. These layers may then be separated from the underlying target material to from sheets, or shaped bodies, or foils. Or, be made an integral part of the target material depending upon the intended application.

The second step in this method involves heating the deposited material or the shaped body to a pore forming temperature sufficient to vaporize the high volatility material powder particles to create a vapor pressure sufficiently high to cause yielding of surrounding matrix metal, and thus forming pores within the matrix metal.

The high volatility material can be selected from among common elements, single and double oxides, fluorides, carbonates, hydrides, sulfides, borides, nitrides, carbides, chlorides and other compounds of common elements.

Placing the deposited body in a cavity die with pre-determined dimensions to restrict its growth during pore formation can also be used as a method of controlling pore size. This approach can also be useful in manufacturing of shaped parts by allowing the growth of the deposited body due to pore formation, in pre-determined dimensions and directions.

After deposition and before pore formation treatment, the deposited body can be deformation processed for conversion to typical industrial metal forms, including bar stock, plate, sheet, foil, wire, forging stock, and rolled or extruded metal shapes. For example bar stock can be cut to desired sizes for forging, and the deposited body can be forged to a semi-finished shape before pore formation treatment. The final machining to finished size and shape can be accomplished either before or after the pore formation.

Deposition may be made onto a shaped hard die. This can produce nearly net shaped product that can later be put through a pore forming treatment.

The deposited body may also be rolled to sheet or foil thickness before pore formation. The latter can be used as filters or separators after pore formation that allows pore-to-pore connection, or pores that traverse the thickness of the foil.

High volatility powder material type is chosen to provide a pore formation temperature, which is less than the solidus temperature of the matrix metal.

In the same preferred deposition approach, the high volatility material chemical composition can be chosen intentionally to allow release of reactive vapors upon heating to pore forming temperature, to form stable compounds, including oxides, carbides, nitrides, and borides, with one or more elemental ingredients of the matrix metal on or near pore surface. When the pore sizes are chosen to be less than 100 microns (micro-meters), and the average pore spacing is less than 10 micro-meters, preferably less than 3 micro-meters, the matrix metal can experience a strengthening effect due to creation of the thermally stable compounds on the surfaces of pores acting as barriers to dislocation movement. This effect may be quite pronounced, and in some cases doubling, tripling, or even quadrupling of the strength of the matrix metal.

The base metal used as a target for the metal being deposited can be either disposed of after the deposition or can be incorporated into the deposited body being manufactured, thus forming a bi-material;

Porous materials thus produced can have very attractive engineering properties by the virtue of having a porous nature. Porosity can lower thermal conductivity, and thus allow the use of these materials as low-conductivity materials or even insulators. Density would be reduced. Similarly, the porous nature of the products produced by the teachings of this invention, would have desirable energy and sound absorption characteristics.

In transparent matrix materials, such as certain plastics and glasses, pores can provide a means to control transmission, absorbance, and reflectance characteristics of the matrix material. Pore size can be controlled to preferentially absorb or reflect electromagnetic radiation or be completely transparent to such radiation that may cover a wide range of the frequency spectrum. For example, technologically important spectral regions of ultraviolet, visible, infrared, and microwave energies may be controlled. This control can be confined to a window of radiation frequencies (or wavelengths) as well.

Pore size uniformity can be controlled by any of the processing variables that include: pore formation temperature (T2), high volatility material type, particle size, and time at pore forming temperature. Allowing the pore formation to take place in a confined space can be another technique to control pore size, especially under isothermal heating conditions. In addition, intentionally allowed thermal gradients within the matrix material can lead to the production of pore size gradients if so desired for better property control.

A further application of the process of this invention would involve manufacture of parts with interior cavities. Here, the cavities may be macro in scale, but the matrix material itself can be strengthened by stable compound formed pore walls, where the pores are very small (less than 100 μm in diameter), and the pore to pore distance is sufficiently small, preferably less than 10 μm, to cause strengthening of the matrix material. Such a process is possible by allowing two different high volatility materials to be distributed within the matrix material. One of the high volatility materials having lower vaporization temperature would form large, macro scale cavities to be formed due to being super heated, while the other high volatility material particles, having higher vaporization temperature, can only develop vapor pressures sufficient to form small sized pores. This is preferred because only the very small pores, when reacted to form stable surface compounds, can act to strengthen the matrix material vie dispersion strengthening mechanism known to metallurgists knowledgeable in the arts. Such small pores are typically not visible to the naked eye, even at high magnifications in an ordinary light microscope. High volatility material particle size may also be manipulated to create relatively large cavities inside matrix material. While the small high volatility material particles form pores that are small enough and with small separation distances to strengthen the matrix material, larger particle groups could form the cavities.

Therefore, another embodiment of this invention starts with distribution of high volatility material particles within matrix material. It is preferred that the matrix material be substantially dense to be able to accommodate vapor pressures to be generated during the pore formation treatment. Such a dense matrix material maybe obtained through several techniques. Thus, the process of distributing may involve a number of methods including one of the following methods modified to introduce fine particles of high volatility material in a dense matrix material:

-   -   Powder consolidation of a powder mixture under pressure,     -   Electrolytic plating,     -   Electro-less plating,     -   Molten droplet spraying,     -   Vapor deposition,     -   Plasma spraying and coating,     -   High-energy rate forming of powder mixture (i.e., explosive         forming, electro-hydraulic forming, cladding, magnetic forming,         etc.)     -   Roll forming of powder mixture,     -   Extruding of powder mixture,     -   Plastic forming,     -   Sintering of metal powders, and of dried ceramic slurries, and         slips,     -   Melting followed by freezing,     -   Other similar methods.

In the powder consolidation approach, which will be used to describe the invention, computer controlled printer carriage like x-y table systems, such as shown in FIG. 6, can be used to distribute the high volatility material particles within the matrix material in accordance with a pre-determined pattern and location in three dimensions. Such powder application schemes are frequently used in “rapid prototyping” techniques. Patterns thus created can be micro in scale as well as macro.

In FIG. 6, the first step of the process of this invention can be accomplished, namely distribution of high volatility material particles within a substantially dense three-dimensional mass of the matrix material powder 61 to create a composite. First, a powder mixture is obtained, by using an applicator 63 holding a reservoir of matrix material powder lays a layer of the matrix powder inside a metal can 62, and another applicator (not shown but having the same carriage arrangement as the applicator 63 shown) applies the high volatility material particles in pre-determined locations on the already laid out matrix powder, in pre-determined amounts. The composite powder mass containing high volatility material particles in precise locations is built up in the vertical direction. The powder applicator 63 is able to travel on a carriage 64 in the X direction, and the carriage 64 is able to travel in the Y direction.

Alternately, the distribution of the high volatility material particles in the same pre-determined pattern may be accomplished on a film of a fugitive compound first, such as a resin or a mixture of cellulose acetate and acetone, using the same fugitive material as a binder, and the film is positioned within the matrix material. Once all of the high volatility material particles have been placed within the matrix material, then the fugitive binder could be removed by heating the assembly to a temperature to volatilize the fugitive binder at a temperature below the pore formation temperature for the high volatility material particles. Matrix material may be in powder form or solid form.

For plastic materials the distribution of the high volatility material particles simply would involve mixing in the liquid thermosetting compounds or with the granules of thermoplastic materials before being molded. Molding can be accomplished by any of the standard molding techniques that include compression molding, injection molding, blow molding, extrusion molding, or vacuum molding.

For ceramic materials, distribution of high volatility material particles could occur while the ceramic powders are still in the slurry stage, and later by drying and sintering to densify the mixture.

For matrix materials that are metals, after all high volatility material particles have been distributed and all matrix material powder is laid out, can 62 is enclosed, degassed, and maybe sealed as a pre caution against oxidation of powders during heating. Alternately, instead of using partial vacuum, which can be generated inside can 62, a protective gaseous atmosphere can also be used. The sealed can 72 and its contents are heated in a furnace to a pre-determined consolidation temperature, and consolidated in a pressing arrangement 70 such as the one shown in FIG. 7. Here, the sealed and heated can 72 is placed inside a cavity die 77, and through the press ram 74 a pressure 75 is applied to consolidate the powder mixture inside can 62. The consolidation pressure 75 is chosen to be sufficiently high to bond the matrix powder particles together to quickly form a fully dense bulk material. Within the consolidated matrix material, high volatility material particles are distributed in a pre-determined pattern in three dimensions. The composite thus produced is substantially dense. The can is then cut up and removed.

As a processing second step, the composite is heated to a pore formation temperature, at which vapor pressure of the high volatility material is higher than the yield strength of said matrix material. Then, the high volatility material particle(s) evaporate to form at least one cavity within said matrix material. This step has been described in detail above.

With the aid of FIGS. 6 & 7 basic processing steps have been described. The intelligent and precise control of basic process control factors can lead to manufacture of very complex internal cavity designs in objects, like metal and plastic parts. Cavity size, shape, relative position within the object, would be determined by the basic process control variables, which include:

-   -   Pore (or cavity) formation temperature (T2)     -   Pore forming, high volatility material         -   Type         -   Particle size         -   Particle shape         -   Amount         -   Particle distribution pattern     -   Mold dimensions and shape     -   Matrix powder particle size and shape     -   Miscellaneous factors, such as: time at pore forming         temperature, changes in pore forming temperature, cooling and/or         heating rate and direction, usage of more than one type of high         volatility material.

These processing variables can be manipulated to manufacture many different products with many complex shapes, with nano meter sized features to sizes beyond decimeters.

FIG. 8 shows a mold 81 that will allow matrix material 83 to grow only in the upper direction toward the open volume 84 available for growth upon heating to form cavities that are restricted by the sides of mold 81, and form cavities displaying cross-sections 99 similar to those shown in FIG. 9 after heating in a furnace 92. Spacing, shape and size of the cavities can be controlled by the size and shape of high volatility material particles 85, and also by the pore formation temperature, which affects the vapor pressure of the evaporated high volatility material. Top surface of the matrix material 95 after pore formation may or may not be smooth, as this would depend on the matrix material's ability to carry its own weight at the pore formation temperature selected. High volatility material particles pre-formed into long string like shapes could grow into long narrow cavities (channels).

Growth of the matrix material 103 under internal vapor pressure can also be limited to a pre-determined amount in order to achieve a desired internal cavity shape or a desired surface morphology. Mold 101 of FIGS. 10 and 11 would only allow growth into the pre-designed mold cavity 104. Once the mold is filled with matrix 103 and the vapor created cavities 119 at pore formation temperature, cavity growth would then stop, and the cavity shapes would conform to the mold surface. Mold cavity would be filled with the available matrix material 113 and with all the pores 119 (cavities) created by vaporizing high volatility material particles.

Growth of matrix 103 under internal vapor pressure would allow the matrix material to conform to interior shape of the mold, which may be a more complex shape as shown in FIG. 12 than the shape of the mold 101 in FIG. 10. In FIG. 13, the complex shaped mold of FIG. 12 is completely filled, resulting in some degree of cavity 138 shape change adjacent to the protrusion 121 of the mold in FIG. 13. Here the cavity walls will tend to be straight, as pressures on both sides of the wall 127 will tend to cancel each other under ideal conditions. Cavity (or channel) shape will largely be defined by the growth restricting mold walls.

Instead of the mold 101 of FIG. 10, if partially open mold 141 of FIG. 14 is used for the growth of a matrix material similarly designed and shaped as that in FIG. 10, the final shape of the matrix material may then resemble the matrix material object 143 shown in FIG. 14. Object 143 has both open 147 and closed 149 cavities depending on whether there is restriction imposed by the near-by rigid walls of the mold 141.

High volatility material materials 155 can be arranged in close proximity of each other, as shown in FIG. 15, within the matrix material so that when grown into a closed mold, upon heating to the pore formation temperature, pores would join to form a pre-designed continuous cavity, or channel system 169 shown in FIG. 16. The channel system could carry gas or fluid. Thus the invention allows the manufacture of many parts with engineered internal cavities, such as heat exchangers, electronic components, or medical devices of variety of shapes and sizes, with channels measuring from nanometers to centimeters or even larger.

High volatility material particles can be put together as strings, needle like performs, or other shapes to improve the manufacturing method thus described. To strengthen the matrix material, nanometer size pores, and thermally stable particles can be mixed with the matrix powder prior to the consolidation step. Such hard particles can act as barriers to dislocation movement, strengthening the matrix material.

Similar strengthening effect can be obtained by creating nano or micro-meter sized pores within the matrix by mixing very small, several nano or micrometer size diameter high volatility material particles during the powder mixing step, and allowing the formation of thermally stable compounds on or near the pore surfaces. These hardened tiny pores in the matrix material of the finished product can act like dispersion strengtheners. To form the thermally stable compounds, high volatility material is selected from elements from the periodic table, oxides, carbides, nitrides, chlorides, bromides, sulfides, fluorides, carbonates, and hydrides of elements. The matrix material can have more than one type of high volatility material powder, with different chemistries, mixed in, or can have more than one size range of particles in order to create large and small pores within the same matrix material. When the matrix material contains particles of at least two types of high volatility material powder, at least one type of the high volatility material types releases thermally stable compound forming vapors upon heating to pore forming temperature; said vapors forming stable compounds including oxides, borides, nitrides, and carbides with one or more elemental ingredients of said matrix metal on or near pore surfaces; covering said surfaces at least in part; said stable compound covered pores strengthening said matrix metal.

Referring to FIG. 17, where matrix material 173 containing a single piece of high volatility material 175 has been placed in a shaped cavity mold 171. Mold cavity defining the exterior shape of the matrix material after pore formation. Upon heating to the pore formation temperature, matrix 173 is expanded to conform to the interior shape of mold 171 by the vapor pressure of high volatility material piece 175. In FIG. 18 expanded matrix material by conforming to the interior of the mold 171 has become a part 183 with hollow interior 189. The invention thus provides a method to manufacture hollow parts from materials, such as metals, metal alloys, plastics, glasses, ceramics, and composite materials. While the mold 171 needs to be two or more pieces, so that the manufactured part can be extracted, the part produced will be a single piece part. Golf club heads and buoyancy spheres (used to raise sunken ships) are two good examples of hollow parts.

In the method thus described, the matrix material can be selected from one of the groups of materials including metals, semi-metals, metal alloys, plastics, glasses, and ceramics. For each material group, high volatility materials may have different vaporization and pore forming temperatures to be suitable for the matrix material.

Pore Forming Materials

The following descriptions relate to the high volatility (pore forming) materials:

-   -   High volatility materials may be elements from the periodic         table,     -   They may be oxides, carbides, nitrides, chlorides, carbonates,         hydrides, bromides, sulfides, and fluorides, and other compounds         of elements from the periodic table.     -   These particles may have two or more chemical compositions         within the same matrix material.     -   They may be selected to vaporize at a desirable temperature         range. At the pore formation temperature these materials should         show significant vapor pressure.     -   They may be selected to vaporize and react with pore walls to         form desirable thermally and chemically stable compounds by         reacting with matrix material at the pore forming temperature,         such oxides, carbides, nitrides, or borides, or form oxides and         other compounds that may be attractive for their surface,         electrical or magnetic properties.     -   particles may be compacted into long string or needle-like         shapes, spheres, or other shapes suitable to promote shaped         cavities within the matrix.

Matrix material may be selected from metals, semi-metals, metal alloys, glasses, ceramics, plastics, or composites. It is preferred that the matrix material is deformable at the pore forming temperature.

A brief survey of the literature¹⁻⁴ reveals many compounds that can be used at various estimated temperature ranges to manufacture porous materials. Some of these compounds are listed in Tables 1 and 2. TABLE 1 Selected Oxides That May Be Useful As Pore Formers Oxide Thermal Stability Useful Range, ° C. Vapor pressure Li₂O₂ Decomposes, gives up 160-350 oxygen Li₂O Sublimes 1150-1400 1200° C., P = 1 Atm. 1400° C., P = 8 Atm. Na₂O Volatile above 1300° C. 1300-1500 2000° C., P = 6 Atm. Na₂O₂ Oxygen loss begins 300-700 at 311° C. NaO₂ Full decomposition 500-650 >540° C. P₂O₅ Sublimes at 359° C. 350-550  359° C., P = 1 Atm.  780° C., P = 100 Atm. KO₂ Full decomposition 500-650 at 543° C. MnO Sublimes at 3400° C. 3350-3500 Co₃O₄ Dissociation  970° C., P = 1.02 × 10⁵ N/m² CuO Decomposes above 800° C.  800-1200 1100° C. 1 × 10⁵ N/m² ZnO Volatilizes fully at 1370-1500 1370-1400° C. GeO Sublimes above 700° C. 700-900 SeO₂ Dissociates readily 1000-1200 above 1000° C. SrO₂ Gives off oxygen at 700-900 700-800° C. MoO₃ Sublimes above 650° C.  800-1100  900° C., P = 1 Atm. 1100° C., P = 100 Atm. CdO Sublimes at 700° C. 1300-1500 1400° C., P = 1 Atm. without melting TeO₂ Sublimes at about 450-600 450° C. BaO₂ Decomposes above 800° C. 800-900 WO₃ Evaporates above 1200-1700 1600° C., P = 1 Atm. 1000° C. OsO₂ Decomposes at 650° C. 600-750 TlO Dissociates at 490° C. 450-900 1 Atm. = 5103 N/m² = 14.696 lbs/in²

TABLE 2 Selected Compounds and Elements That May Be Useful As Pore Formers Solid Useful Compound Comments Range, ° C. Vapor Pressure WCl₆ Sublimes 300-500  350° C., P = 1 Atm. ZrCl₄ Sublimes 300-500  330° C., P = 1 Atm. CrCl₃ Sublimes  900-1100  900° C., P = 1 Atm. AgCl₂ Sublimes 1550-1700 1600° C., P = 1 Atm. ScCl₃ Sublimes  900-1100  950° C., P = 1 Atm. Al₂F₆ Sublimes 1200-1350 1250° C., P = 1 Atm. ZrF₄ Sublimes  850-1100  900° C., P = 1 Atm. IrF₆ Sublimes  50-120  54° C., P = 1 Atm UF₆ Sublimes  50-120  56° C., P = 1 Atm InBr₃ Sublimes 260-350  270° C., P = 1 Atm ScBr₃ Sublimes  850-1100  900° C., P = 1 Atm. ZrBr₄ Sublimes 300-500  350° C., P = 1 Atm. Al₂Cl₆ Sublimes 160-270  180° C., P = 1 Atm CrCl₂ Sublimes 1100-1250 1130° C., P = 1 Atm ZrCl₄ Sublimes 300-400  320° C., P = 1 Atm WOCl₄ Sublimes 210-310  230° C., P = 1 Atm Ba Boiling 1600-1800 1700° C., P = 1 Atm. point Cd Boiling 750-850  767° C., P = 1 Atm. point Cs Boiling 650-850  670° C., P = 1 Atm. point Cu Boiling 2700-3000 2727° C., P = 1 Atm point Fe Boiling 2900-3100 2920° C., P = 1 Atm point I Boiling 150-250  180° C., P = 1 Atm. point Li Boiling 1350-1500 1400° C., P = 1 Atm. point Mg Boiling 1060-1220 1110° C., P = 1 Atm. point P Boiling 250-400  280° C., P = 1 Atm. point K Boiling 710-850  760° C., P = 1 Atm. point Na Boiling  850-1000  880° C., P = 1 Atm. point Sr Boiling 1340-1500 1370° C., P = 1 Atm. point S Boiling 400-550  445° C., P = 1 Atm. point Zn Boiling  860-1025  907° C., P = 1 Atm. point “Useful temperature ranges” indicated in the two tables are estimates based on the vapor pressures obtained from literature and the assumed necessity for vapor pressures being equal to or higher than one atmosphere. These lists are by no means complete, and certainly there are other elements and compounds such as sulfides, bromides, carbides, carbonates, hydrides that can be used in creating porous structures by virtue of their high vapor pressures at temperatures that the strength of the matrix material becomes lower than the vapor pressure of the high vapor pressure material particles.

A review of the lists in Tables 1 & 2 points to a number of candidate pore forming compounds from among the compounds listed for several matrix material classes:

For aluminum alloys: L₂O₂, Na₂O₂, NaO₂, P₂O₅, KO₂, TeO₂, TlO, WCl₆, ZrCl₄, ZrBr₄, Cs, K, Na

For Steels: Li₂O, CuO, ZnO, GeO, SeO₂, MoO₃, CdO, WO₃, CrCl₃, ScCl₃, Al₂F₆, ZrF₄, ScBr₃, Li, Mg, Sr

For Copper Alloys: GeO, Co₃O₄, SrO₂, MoO₃, BaO₂, CrCl₃, ScCl₃, ZrF₄, ScBr₄, Cd, Sc, K, Na

For Plastics: Li₂O₂, Al₂Cl₆, WOCl₄, I

For Glasses & Ceramics: Na₂O, MnO, CdO, AgCl₂, Al₂F₆, Fe, Li, Sr

EXAMPLES Example 1

Pure copper powder, with an average particle size of 4 μm, was mixed with pure MoO₃ powder, average particle diameter 40 nano meters. The MoO₃ powder was about 2% by weight (3.8% by volume). The mixture was placed in a cylindrical die between two loose graphite powder layers, and heated inside a furnace to 550° C. and pressed until the copper powder was at least 90% dense. The consolidation temperature of 550° C. is below the vaporization temperature of MoO₃ (see Table 1). The graphite powders provided a reducing atmosphere of CO to prevent oxidation of the copper powder during heating. This arrangement also provided isothermal conditions for the hot pressing of the powders inside the furnace. After measurements of the dimensions, the piece was buried under a mass of graphite powder, which provided isothermal heating under the cover of CO atmosphere, to the pore forming temperature of 985° C. This temperature is about 98° C. below the melting point of copper. The piece was cooled after holding at the pore forming temperature for about a minute. Final dimensional measurements of the sample indicated that the original consolidated volume of the copper piece had been increased by 15.8%. The sample showed no visible signs of pores. Calculated average pore diameter was 69.1 nano meters, and the average distance between pores was 1053 nano meters (1.053 micro meters). Decreasing the size of MoO₃ particles or MoO₃ amount added could reduce the distance between pores. For example, If the amount of high volatility material (MoO₃) added was increased by a factor of two, and the particle size was reduced to 20 nano meters, then the average distance between the pores would be 263 nano meters or 0.263 micro meters.

Example 2

Pure copper powder, with an average particle size of 4 μm, was mixed with pure MoO₃ powder, average particle diameter 40 nano meters. The MoO₃ powder was about 3.5% by weight. The mixture was placed in a cylindrical die between two loose graphite powder layers, and heated to 560° C. and pressed until the copper powder was at least 90% dense. The consolidation temperature of 560° C. is below the vaporization temperature of MoO₃ (see Table 1). The graphite powders provided a reducing atmosphere of CO to prevent oxidation of the copper powder during heating. This arrangement also provided isothermal conditions for the hot pressing of the powders inside the furnace. Then the pressure was released, and the piece was cooled to room temperature. After measurements of the dimensions, the piece was buried under a mass of graphite powder, which provided isothermal heating under the cover of CO atmosphere, to the pore forming temperature of 990° C. This temperature is about 9.3° C. below the melting point of copper. The piece was cooled after holding at the pore forming temperature for about a minute. Dimensional measurements of the final piece indicated that the original consolidated volume of the copper piece had been increased by about 39%. And, the final density of the copper piece was now 64.2% of its theoretical density. The copper piece had several small yet visible pores on its surface, seen as blisters. Calculated average pore diameter was 76 nano meters, and the average distance between pores was 601 nano meters (0.601 micro meters). Obviously some of the close proximity pores joined to form larger pores that were visible to the naked eye at the piece surface. Similar pore joining would be expected for the pores forming in the interior of the piece. This could be eliminated or minimized by through mixing of the powders.

Example 3

Pure copper powder, with an average particle size of 4 μm, was mixed with pure MoO₃ powder, average particle diameter 40 nano meters. The MoO₃ powder was about 3% by weight. The mixture was placed in a cylindrical die between two loose graphite powder layers, and heated to 560° C. and pressed until the copper powder was at least 90% dense. The consolidation temperature of 560° C. is below the vaporization temperature of MoO₃ (see Table 1). The graphite powders provided a reducing atmosphere of CO to prevent oxidation of the copper powder during heating. This arrangement also provided isothermal conditions for the hot pressing of the powders inside the furnace. Then the pressure was released, and the piece was cooled to room temperature. After measurements of the dimensions, the piece was buried under a mass of graphite powder, which provided isothermal heating under the cover of CO atmosphere, to the pore forming temperature of 985° C. This temperature is about 98° C. below the melting point of copper. The piece was cooled after holding at the pore forming temperature for about a minute. Dimensional measurements of the final piece indicated that the original consolidated volume of the copper piece had been increased by 27.3%. The copper piece had only few visible pores on its surface, seen as blisters. Calculated average pore diameter was 71.8 nano meters, and the average distance between pores was 702 nano meters. Obviously some of the close proximity pores joined to form larger pores that were visible to the naked eye at the piece surface. Similar pore joining would be expected for the pores forming in the interior of the piece. This could be eliminated or minimized by through mixing of the powders.

Example 4

Slightly oxidized copper powder, gray in color, and having −325 mesh particle size, was mixed with pure MoO₃ powder, average particle diameter 40 nano meters. The MoO₃ powder was about 3.2 % by weight. The mixture was placed in a cylindrical die between two loose graphite powder layers, and heated to 593° C. and pressed until the copper powder was at least 90% dense. The consolidation temperature of 593° C. is below the vaporization temperature of MoO₃ (see Table 1). The graphite powders provided a reducing atmosphere of CO to prevent oxidation of the copper powder during heating. This arrangement also provided isothermal conditions for the hot pressing of the powders inside the furnace. Then the pressure was released, and the piece was cooled to room temperature. After measurements of the dimensions, the piece was buried under a mass of graphite powder, which provided isothermal heating under the cover of CO atmosphere, to the pore forming temperature of 1032° C., and held at that temperature for about 5 minutes. This temperature is about 51° C. below the melting point of copper. The piece was cooled; and dimensional measurements of the final piece indicated that the original consolidated volume of the copper piece had been increased by 56.7%. The copper piece had numerous visible pores on its surface, seen as blisters. This could be eliminated or minimized by through mixing of the powders. Calculated average pore size was 87.1 nano meters, and the average distance between pores was 658 nano meters (0.658 micro meters).

Example 5

Powder of a copper based precipitation hardening alloy with a composition of Cu-3Ag-0.5Zr, and having −325 mesh particle size, was mixed with pure MoO₃ powder, average particle diameter 40 nano meters. The MoO₃ powder was about 3.2 % by weight. The mixture was placed in a cylindrical die between two loose graphite powder layers, and heated to 571° C. and pressed until the copper powder was at least 90% dense. The consolidation temperature of 571° C. is below the vaporization temperature of MoO₃ (see Table 1). The graphite powders provided a reducing atmosphere of CO to prevent oxidation of the copper powder during heating. This arrangement also provided isothermal conditions for the hot pressing of the powders inside the furnace. Then the pressure was released, and the piece was cooled to room temperature. After measurements of the dimensions, the piece was heated under the cover of CO atmosphere, to the pore forming temperature of 1000° C. And the piece was cooled to room temperature. Dimensional measurements of the pore formed piece indicated that the original consolidated volume of the copper piece had been increased by 14.5%. At this point no pores were visible on the surface. The piece was put back in the furnace and heated to 1050° C. This temperature is about 33° C. below the melting point of copper. The piece was cooled after holding at the pore forming temperature for about a minute. Dimensional measurements of the final piece indicated that the original consolidated volume of the copper piece had been increased by only 18.9%. The copper piece had no visible pores on its surface. Here, the first heating to a pore forming temperature of 1000° C. probably produced pores that were oxidized leaving very little MoO₃ for vapor formation for the second heating to a pore formation temperature of 1050° C. While this pore forming temperature is higher than the first, pore volume probably increased but only slightly due to unavailability of MoO₃ vapor pressure. Zirconium element present in the copper alloy would be expected to reduce the MoO₃ vapors to produce ZrO₂, an oxide ceramic, which is thermally stable up to its melting point of 2,700° C. Formation of ZrO₂ is favored thermodynamically², over the formation of MoO₃. Formation of ZrO₂ made it difficult to expand the pore when heated for the second time. The volume of the ZrO₂ that formed is larger than the volume of the Zirconium atoms it replaced, by a factor of 1.56 according to the calculations of Kubaschevski (Reference No. 4, p. 9-14). This volume increase creates a stress field around the pores. The final average pore size was 72.6 nano meters, and the average distance between pores were 658 nano meters (0.658 micro meters).

Example 6

Pure copper powder, with an average particle size of 4 μm, was mixed with ZrF₄ powder with an average particle size of 25 nano meters. The ZrF₄ powder was about 1.6% by weight (3.245% by volume). The mixture was placed in a cylindrical die between two loose graphite powder layers, and heated to 550° C. and pressed until the copper powder was at least 90% dense. The consolidation temperature of 550° C. is below the vaporization temperature of ZrF₄ (see Table 2). The graphite powders provided a reducing atmosphere of CO to prevent oxidation of the copper powder during heating. This arrangement also provided isothermal conditions for the hot pressing of the powders inside the furnace. Then the pressure was released, and the temperature was increased to the pore forming temperature of 985° C. This temperature is about 98° C. below the melting point of copper. The piece was cooled after holding at the pore forming temperature for about two minutes. Dimensional measurements of the final piece indicated that the original consolidated volume of the copper piece had been increased by 18.8%. The piece showed no visible signs of pores. Calculated average pore diameter was 75.8 nano meters (0.0758 micro meters), and the average distance between pores was 770 nano meters (0.770 micro meters). In this case, there is no thermally stable oxide formation on pore walls is expected.

These experiments showed feasibility of forming pores in materials as described above, and showed that thermally stable compounds could be formed on pore walls, and that the size of pores and the distance between pores could be predetermined to produce strengthening and other property effects as well as reductions in density of these materials.

Although in the above descriptions of the processes of this invention creation of porous metals, and dispersion strengthening via stable compound formation on pore walls have been the focus, processes to form porous materials other than metals, such as plastics, glasses, and ceramics, would not differ greatly. 

1. A powder metallurgy method of forming a lightweight porous material body, comprising: a) mixing matrix material powder with one or more types of high volatility material powder having tendency to vaporize when heated to its vaporization temperature, b) hot consolidating the mixture under sufficient pressure and at a temperature below vaporization temperature of high volatility material powder to form a substantially consolidated body consisting of dispersions of high volatility material powder particles in said matrix material, c) heating said consolidated body to a temperature sufficient to vaporize said high volatility material powder particles and to create a vapor pressure sufficiently high to cause yielding of surrounding matrix material, thus forming pores, d) cooling said compact body after pore formation.
 2. The method of claim 1 wherein said consolidated body is heated in step (c) to a pore forming temperature below the solidus temperature of matrix material for better control of pore size and size distribution.
 3. The method of claim 1 wherein said consolidated body is heated in step (c) to a pore forming temperature below the liquidus temperature of matrix material.
 4. The method of claim 1 wherein said consolidated body is heated in step (c) to a pore forming temperature above the liquidus temperature of matrix material.
 5. The method of claim 1 wherein said high volatility material is selected from a group of compounds including common elements, oxides, fluorides, borides, carbonates, carbides, hydrides, and chlorides of elements.
 6. The method of claim 1 wherein said consolidation takes place isothermally.
 7. The method of claim 1 wherein said consolidation takes place under a protective atmosphere, including partial vacuum.
 8. The method of claim 1 wherein said consolidated body is placed in step (c) in a cavity die to restrict its growth due to pore formation, in at least one direction.
 9. The method of claim 1 wherein said consolidated body is deformation processed for easy conversion to typical industrial material forms, including bar stock, plate, sheet, foil, wire, forging stock, and rolled or extruded material shapes, prior to carrying out step (c).
 10. The method of claim 1 wherein said consolidation in step (b) produces a net or near net shaped product prior to forming pores in step (c).
 11. The method of claim 1 wherein said high volatility material particles release thermally stable compound forming vapors upon heating to pore forming temperature; forming stable compounds including oxides, borides, nitrides, carbides with one or more elemental ingredients of said matrix metal on or near pore surfaces; covering said surfaces at least in part; said stable compound covered pores strengthening said matrix metal.
 12. The method of claim 11 wherein average separation distance between said stable compound formed pores is less than 10 micro-meters.
 13. The method of claim 11 wherein average diameter of said stable compound covered pores is less than 100 micro-meters.
 14. The method of claim 1 wherein said powder mixture is placed at least at one surface next to another powder mass devoid of high volatility material powder, prior to consolidation; consolidation causing bonding of said adjoined powder masses.
 15. The method of claim 1 wherein said powder mixture is placed at least at one surface next to a rigid material mass devoid of high volatility material, prior to consolidation; consolidation causing bonding of said mixture and said rigid material;
 16. The method of claim 1 wherein said consolidated body has a density higher than 90% of its theoretical density.
 17. A deposition method of forming a lightweight porous metal body, comprising: a) dispersing one or more types of high volatility material powder within a matrix metal being deposited onto a base material; high volatility material powder having tendency to vaporize when heated to its vaporization temperature to create a deposited body made of matrix material, wherein high volatility powder is distributed, b) heating said deposited body to a pore forming temperature sufficient to vaporize said high volatility material powder particles and to create a vapor pressure sufficiently high to cause yielding of surrounding matrix metal, and thus forming pores within said matrix metal.
 18. The method of claim 17 wherein said deposited body is heated in step (b) to a pore forming temperature below the solidus temperature of matrix metal, forming pores within said matrix metal.
 19. The method of claim 17 wherein said deposition method includes the methods of plating, vapor deposition, plasma spraying, molten droplet spraying.
 20. The method of claim 17 wherein said high volatility material is selected from a group of compounds including common elements, oxides, fluorides, borides, carbonates, carbides, hydrides, and chlorides of elements.
 21. The method of claim 17 wherein said deposited body is placed in step (b) in a cavity die to restrict its growth due to pore formation, in at least one direction.
 22. The method of claim 17 wherein said deposited body is deformation processed for conversion to typical industrial metal forms, including bar stock, plate, sheet, foil, wire, forging stock, and rolled or extruded metal shapes, prior to carrying out step (b).
 23. The method of claim 17 wherein said deposited metal bonds to said base material producing a bi-material as semi finished product.
 24. The method of claim 17 wherein said base material is disposable.
 25. The method of claim 17 wherein said high volatility material particles release thermally stable compound forming vapors upon heating to pore forming temperature, forming stable compounds including oxides, borides, nitrides, and carbides with one or more elemental ingredients of said matrix metal on or near pore surfaces, covering said surfaces at least in part; said stable compound covered pores strengthening said matrix metal.
 26. The method of claim 25 wherein mean separation distance between said stable pores is less than 10 micro-meters.
 27. The method of claim 25 wherein average diameter of said stable compound covered pores is less than 100 micro-meters.
 28. A process of manufacturing objects containing at least one pore, the process comprising the steps of: c) distributing at least one high volatility material powder particle within a substantially dense matrix material to create a composite, d) heating said composite to a pore formation temperature, at which vapor pressure of said high volatility material is higher than the yield strength of said matrix material, wherein said high volatility material particle evaporates to form at least one cavity pore within said matrix material.
 29. The process of claim 28 wherein said distributing involves one of the following methods modified to introduce particles of high volatility material in said matrix material: consolidation of powder mixture under pressure, electrolytic plating, electroless plating, vapor deposition, molten droplet deposition, plasma coating, plasma spraying, high-energy forming methods, roll forming, extruding, plastic forming methods, slurry mixing and drying, sintering of dried ceramic slips and slurries, melting, and other similar methods.
 30. The process of claim 28 wherein said high volatility material is selected from elements from the periodic table, oxides, carbides, nitrides, chlorides, bromides, sulfides, fluorides, carbonates, and hydrides of elements.
 31. The process of claim 28 wherein said composite is placed inside a shaped mold before step (b); said shaped mold defining, at least in part, the exterior shape of said matrix material after step (b).
 32. The process of claim 28 wherein said high volatility material particle consists of more than one powder particle pre-formed into a shape to promote shaped cavities within said matrix material.
 33. The process of claim 28 wherein chemistry of at least some of said high volatility material particles differ from each other.
 34. The process of claim 28 wherein at least some of said high volatility material particles evaporate and react with pore walls of said matrix material to form at least one type of thermally stable compound at said pore forming temperature, thermally stable compound formation strengthening said matrix material.
 35. The process of claim 28 wherein at least some of said particles are distributed in a pre-determined pattern, in selected location within said matrix material.
 36. The process of claim 28 wherein said high volatility material particles are placed in close proximity of each other, said pores forming in close proximity and joining to form a continuous cavity within said matrix material when heated to pore formation temperature.
 37. The process of claim 28 wherein size, shape, and relative position of said pore cavity within said matrix material would be determined by any combination of the basic process control variables, variables including pore formation temperature, type, size, amount, shape, and particle distribution pattern of high volatility material, mold dimensions and shape, matrix powder particle size and shape, and other factors, including time at pore forming temperature, changes in pore forming temperature, cooling and heating rate and direction, usage of more than one type of high volatility material.
 38. The process of claim 28 wherein said composite contains particles of at least two types of high volatility material powder wherein at least one type of said high volatility material particles release thermally stable compound forming vapors upon heating to pore forming temperature, said vapors forming stable compounds including oxides, borides, nitrides, and carbides with one or more elemental ingredients of said matrix metal on or near pore surfaces covering said surfaces at least in part, said stable compound covered pores strengthening said matrix metal.
 39. Products produced according to claims 1, or 17, or
 28. 40. Products produced according to claims 1, 17, or 28, wherein at least some of said pores are interconnecting.
 41. A process according to claims 1, 17, or 28, wherein said pore size is controlled by a combination of processing variables, including pore forming temperature and time at pore forming temperature, high volatility material type and particle size, and confinement of said matrix material, at least in one direction, to a pre-determined volume during pore formation.
 42. The process of claim 41 wherein said matrix material is a brittle material including a ceramic, a glass, or a brittle metal, wherein presence of said pores improves ductility of said matrix material.
 43. A process according to claims 1 or 28, wherein said matrix material is a transparent material, including some plastics and glasses wherein size of said pores is controlled to impart desired radiation energy transmission, absorption, reflection properties to said transparent material.
 44. Products produced by a method according to claims 1, 17, or 28, wherein the product has one or more of desirable engineering characteristics, including high energy absorption properties, sound absorption properties, low thermal conductivity properties, and large surface area per unit volume.
 45. A process according to claims 1, 17, or 28, wherein said matrix material is selected from one of the groups of materials including metals, semi-metals, metal alloys, plastics, glasses, ceramics, and any combination of these materials.
 46. The method of claim 17 wherein said dispersing and heating steps are controlled to produced pore sizes to be less than 100 microns.
 47. The method of claim 17 wherein said dispersing and heating steps are controlled to produce average pore spacing to be less than 10 micrometers.
 48. The method of claim 17 wherein said dispersing and heating steps are controlled to produce average pore spacing to be less than 3 micrometers. 